This invention relates to oxygen-consuming electrochemical batteries and cells with improved high rate and high power discharge capability, particularly during intermittent use.
Electrochemical battery cells that use oxygen from outside the cell as an active material to produce electrical energy can be used to power a variety of portable electronic devices. Oxygen enters the cell, where it can be used as an active material in the oxygen consuming (e.g., positive) electrode, where the oxygen consuming electrode promotes the reaction of the oxygen with the cell electrolyte and, ultimately, oxidation of the counter (e.g., negative) electrode active material. The material in the oxygen consuming electrode that promotes the reaction of oxygen with the electrolyte is often referred to as a catalyst. However, some materials used in these electrodes are not true catalysts because they can be at least partially reduced, particularly during periods of relatively high rate cell discharge.
One example of an oxygen consuming cell is an oxygen-depolarized metal/air cell in which air from outside the cell enters the cell, and the oxygen is reduced by the positive, or air, electrode. The negative electrode active material is a metal, such as zinc, that is oxidized during cell discharge. A metal/air cell generally has an aqueous alkaline (e.g., KOH) electrolyte. Another example of an oxygen consuming cells is a fuel cell.
An advantage of an oxygen consuming cell such as a metal/air cell is its high energy density, since at least a portion of the active material of the positive electrode comes from outside the cell, thereby either reducing the cell volume or making more volume available for the negative electrode active material.
Oxygen consuming cells can have a maximum discharge rate. The maximum discharge rate can limited by the rate at which oxygen can enter the oxygen consuming electrode. In the past, efforts have been made to increase the rate of oxygen entry into cell and/or limit the rate of entry of undesirable gases, such as carbon dioxide, that can cause undesirable reactions, as well as limit the rate of water entry or loss that can fill void space in the cell intended to accommodate the increased volume of discharge reaction products or dry the cell out, respectively. Examples of these approaches can be found in U.S. Pat. No. 6,558,828; U.S. Pat. No. 6,492,046; U.S. Pat. No. 5,795,667; U.S. Pat. No. 5,733,676; U.S. Patent Publication No. 2002/0150814; and International Patent Publication No. WO02/35641. However, changing the diffusion rate of one of these gases generally affects the others as well. Even when efforts have been made to balance the need for a high rate of oxygen diffusion and low rates of CO2 and water diffusion, there has been only limited success.
At higher discharge rates, it is more important to get sufficient oxygen into the oxygen reduction electrode, but during periods of lower discharge rates and periods of time when the cell is not in use, the importance of minimizing CO2 and water diffusion increases. To provide an increase in air flow into the cell only during periods of high rate discharge, fans and pumps have been used to force air into cells (e.g., U.S. Pat. Nos. 6,500,575 and 6,641,947 and U.S. Patent Publication No. 2003/0186099), but such air movers and controls for them can add cost and complexity to manufacturing, and fans, even micro fans, can take up valuable volume within individual cells, multiple cell battery packs and devices.
Yet another approach has been to use a water impermeable membrane between an oxygen reduction electrode and the outside environment having flaps that can open and close as a result of a differential in air pressure, e.g., resulting from a consumption of oxygen when the battery is discharging (e.g., U.S. Patent Publication No. 2003/0049508). However, the pressure differential may be small and can be affected by atmospheric conditions outside the battery.
Commonly assigned U.S. Patent Publication No. 2008/0254345 discloses a valve that is operated by an actuator that responds to changes in a potential applied across the actuator to open and close the valve. The rate at which oxygen can enter the cell is minimized when the cell is not being used to provide power, but the valve opens and the rate of oxygen entry into the cell increases as the demand for power increases.
All of these prior art approaches have one or more disadvantages. For example, it may be necessary to compromise by limiting either high rate discharge performance in order to minimize cell deterioration during periods of no or low rate discharge or the total useful life of the cell in order to provide the desired high rate discharge capability. Valves, fans and electronic controls can be used to increase the maximum discharge rate, but these components can add complexity and cost, use a portion of the cell capacity to operate these components, and take up valuable space within cells, batteries or devices being powered by the batteries.
It is an object of the invention to provide an oxygen-depolarized battery, containing one or more air depolarized cells, such as metal/air cells, that has excellent high rate and high power discharge capability. It is also an object of the invention to provide an improved oxygen-depolarized battery with minimal added complexity and cost, that consumes minimal discharge capacity, and takes little if any additional space within the cell, battery or device in which the battery is used. It is a further object of the invention to provide a means of improving the discharge rate capability of a battery that can be used in combination with other desired cell, battery and device features and controls, such as air managers for controlling the movement of oxygen and other gases into and out of the cell.